1. Field of the Invention
The present invention relates to magnetic sensors such as a spin-valve type thin film element mounted on hard disc devices or the like, and more particularly, relates to a magnetic sensor in which an exchange coupling region between a fixed magnetic layer and an antiferromagnetic layer is specifically defined.
2. Description of the Related Art
In a related spin-valve type magnetic sensor, a free magnetic layer, a nonmagnetic material layer, a fixed magnetic layer, and an antiferromagnetic layer are provided to form a multilayer structure, and the magnetization of the fixed magnetic layer is fixed in a predetermined direction. In addition, a vertical bias magnetic field is applied to each side of the free magnetic layer, and the direction of magnetization thereof is oriented so as to cross that of the fixed magnetic layer. In this type of magnetic sensor, a lower shield layer and an upper shield layer are provided at the top and the bottom of the multilayer structure with gap layers therebetween, and the distance between the lower shield layer and the upper shield layer determines the resolution of detecting an external magnetic field.
However, heretofore, since the antiferromagnetic layer is provided over the entire fixed magnetic layer, the thickness of the multilayer structure is increased in the layered direction. Hence, the distance between the upper shield layer and the lower shield layer is increased, and as a result, there is a limit to improve the resolution.
Accordingly, in a magnetic sensor disclosed in Japanese Unexamined Patent Application Publication No. 2000-163717, a thin part is formed at a central portion of an antiferromagnetic layer in a width direction. The thin part described above is formed so as to decrease the distance between shield layers provided at the top and the bottom.
Since the thin part is formed at the central portion of the antiferromagnetic layer, the distance between the shield layers can be advantageously decreased. However, since being formed so as to fix the direction of magnetization of a fixed magnetic layer, the thin part must have a certain level of thickness. Hence, a relatively large sense current cannot be prevented from being shunted from an electrode layer to the thin part, and as a result, a current loss occurs, thereby decreasing reproduction output.
In addition, in a magnetic sensor in which an antiferromagnetic layer is provided on a fixed magnetic layer, when a transient current flows from an electrode layer by electrostatic discharge (ESD), heat is generated in the element, and the temperature thereof may be increased close to a blocking temperature of the antiferromagnetic layer in some cases. In this case, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-163717, when a thin antiferromagnetic layer, which generates exchange coupling, is present at the central portion of the fixed magnetic layer in the track width direction, the exchange coupling with the fixed magnetic layer at this thin part becomes unstable.
In particular, when the fixed magnetic layer is formed of a single magnetic layer, the magnetization may not be tightly fixed in many cases. Even in a synthetic ferrimagnetic structure in which the fixed magnetic layer has a first and a second magnetic layer, which are magnetized in an antiparallel state by the RKKY coupling, when one of the magnetic layers is formed so as to be in contact with the antiferromagnetic layer, and the exchange coupling with the antiferromagnetic layer becomes unstable, the direction of magnetization of the magnetic layer in contact with the antiferromagnetic layer is reversed, and as a result, the fixed magnetization of the fixed magnetic layer becomes unstable.
As described above, according to the magnetic sensor disclosed in Japanese Unexamined Patent Application Publication No. 2000-163717, since the antiferromagnetic layer in a magnetic sensing region is intentionally formed thin, a phenomenon in which the fixed magnetization of the fixed magnetic layer becomes unstable as described above cannot be avoided.
In addition, a magnetic sensor disclosed in Japanese Unexamined Patent Application Publication No. 8-7235 has a buffer layer formed of tantalum (Ta) as an underlayer and a pinned ferromagnetic layer provided thereon. The pinned ferromagnetic layer has a multilayer structure composed of a first cobalt (Co) film, a second cobalt (Co) film, and a ruthenium (Ru) film provided therebetween. The magnetizations of the first Co film and the second Co film are fixed by individual anisotropic magnetic fields. The first Co film and the second Co film are antiferromagnetically coupled with each other and are magnetized in the directions antiparallel to each other.
According to this magnetic sensor, an antiferromagnetic layer for fixing the magnetization of the pinned ferromagnetic layer is not provided, unlike that disclosed in Japanese Unexamined Patent Application Publication No. 2000-163713. Hence, compared to the case of Japanese Unexamined Patent Application Publication No. 2000-163713, the shunt loss can be decreased.
However, in the structure in which Co films are provided on a buffer layer composed tantalum, as described above in Japanese Unexamined Patent Application Publication No. 8-7235, it was found that the direction of magnetization of the pinned ferromagnetic layer cannot be appropriately fixed. This problem is also described in Japanese Unexamined Patent Application Publication No. 2000-113418.
A magnetic sensor disclosed in Japanese Unexamined Patent Application Publication No. 2000-113418 solves the problem which occurs in the magnetic sensor in Japanese Unexamined Patent Application Publication No. 8-7235. In this magnetic sensor, the ferromagnetic layer of a laminated ferri-fixed layer is formed of CoFe or CoFeNi, thereby improving the induced anisotropy.
In Japanese Unexamined Patent Application Publication No. 2000-113418, the structure in which an underlayer composed of Ta is provided under the laminated ferri-fixed layer is also disclosed; however according to the experimental results (FIGS. 4 to 7 of Japanese Unexamined Patent Application Publication No. 2000-113418) obtained from two cases in which the Ta underlayer is provided and is not provided, when a CoFe alloy is used for the ferromagnetic layer, the coercive force and the change in magnetoresistance are both increased when the Ta underlayer is not provided.
According to the description in Japanese Unexamined Patent Application Publication No. 2000-113418, in order to increase the induced anisotropy of the laminated ferri-fixed layer, a CoFe alloy is used for the ferromagnetic layer, and the magnetostriction thereof is set to positive.
In order to fix the magnetization of a self-pinning type fixed magnetic layer, the most important factor is uniaxial anisotropy derived from magnetoelastic energy of the fixed magnetic layer. In particular, the optimization of the magnetostriction of the fixed magnetic layer is most important. However, in Japanese Unexamined Patent Application Publication No. 2000-113418, the mechanism for optimizing the magnetostriction of the fixed magnetic layer is not discussed, and a particular structure for optimizing the magnetostriction of the fixed magnetic layer is not described at all.
As described above, in the past, the structure has not been available in which the magnetization of the fixed magnetic layer can be tightly fixed, the reproduction output can be improved, a narrower gap can be achieved, and electrostatic damage can be appropriately suppressed.
In addition, in a related spin-valve type magnetic sensor, since a vertical bias layer is provided for stabilizing the direction of magnetization of the free magnetic layer, the multilayer structure thereof is complicated, and as a result, the manufacturing cost is increased.